The present invention generally relates to induction heating. More particularly, the present invention relates to an induction heating system and a method for controlling a process temperature for induction heating of a workpiece.
Induction heating may be used in multiple different manufacturing processes or steps, e.g., to bond, to cure, to harden or soften metals or other conductive or non-conductive materials.
In a basic induction heating setup, a power supply provides and sends an alternating current to and through an inductor. The inductor is often formed as a coil, for example, a copper coil. In induction heating, typically, a source of high frequency electricity is used to drive an alternating current through such a coil. This coil is often referred to as induction coil or work coil. The passage of current through this coil generates a changing magnetic field (which may be referred to as an alternating magnetic field) in the space within and around the work coil. Depending on the applied alternating current, the magnetic field may be (very) intense and rapidly changing.
In case of direct induction heating, a workpiece to be heated can be placed within this (intense) alternating magnetic field. Such direct induction heating works with conductive materials like metals. Plastics and other non-conductive materials can be heated indirectly by first heating a conductive (metal) susceptor which transfers generated heat to the non-conductive material. In this case, the susceptor to be heated can be placed within the (intense) alternating magnetic field and the heat generated by the susceptor can then be transferred to the non-conductive workpiece.
In direct induction heating, the heating of the workpiece can be referred to as a non-contact heating process. In indirect induction heating, the heating of the susceptor (load) can be referred to as a non-contact heating process. Since it is non-contact, the heating process does not contaminate the material being heated (either the workpiece or the susceptor). It is also very efficient since the heat is actually generated inside the workpiece (direct heating) or the susceptor (indirect heating).
In case of the workpiece being conductive, the alternating magnetic field induces a current flow in the conductive workpiece. The induced current(s) is/are normally known as eddy current(s). When the workpiece is a metal part, (circulating) eddy currents are induced within the part by means of the magnetic field. These eddy currents flow against the electrical resistivity of the metal, generating precise and localized heat without any direct contact between the part and the inductor. This heating occurs with both magnetic and non-magnetic parts, and is often referred to as the “Joule effect”, referring to Joule's first law—a scientific formula expressing the relationship between heat produced by electrical current passed through a conductor.
For ferri- and ferromagnetic materials, e.g., ferrous metals like iron and some types of steel, there is an additional heating mechanism that takes place at the same time as the eddy currents mentioned above. The (intense) alternating magnetic field inside the work coil repeatedly magnetizes and de-magnetizes such magnetic materials and thereby causes magnetic domains to change their direction. This (rapid) flipping of the magnetic domains causes considerable friction and thus produces heat inside the material. Heating due to this mechanism is known as hysteresis loss, hysteresis effect or, in short, hysteresis. In consequence, additional heat is produced within magnetic parts through hysteresis. The hysteresis effect can be a large contributing factor to the heat generated during induction heating, but only takes place inside ferri- and ferromagnetic materials, e.g., ferrous materials. For this reason, ferrous materials lend themselves more easily to heating by induction than non-ferrous materials. Thus, in view of the hysteresis effect, it is easier to heat magnetic materials.
To sum up the above: In addition to the heat induced by eddy currents, magnetic materials also produce heat through the hysteresis effect (described above). This effect ceases to occur at temperatures above the so-called “Curie” point or Curie temperature—the temperature at which a magnetic material loses its ferri- or ferromagnetic properties and becomes paramagnetic. For example, steel loses its ferromagnetic properties when heated above approximately 700° C. This temperature is known as the Curie temperature of steel. This means that above 700° C. there can be no heating of the material due to hysteresis losses. Any further heating of the material must be due to induced eddy currents alone or possible other effects.
In the manufacturing lines for carbon-fiber-reinforced polymer (CFRP) workpieces (CFRP is also sometimes referred to as carbon-fiber-reinforced plastic or carbon-fiber reinforced thermoplastic or often simply carbon fiber and sometimes abbreviated as CRP or CFRTP), one crucial point is the process temperature control and temperature management. Fur curing processes, for example, it is required that the temperature distribution within the CFRP component is nearly uniform. Moreover, the local temperatures should not exceed a critical temperature which would lead to irreversible damage or should not undershoot temperatures which are not sufficient for a reliable curing process. The heating of CFRP components is usually achieved by autoclave convection heating or by direct heating using resistance heating elements or fluid elements. However, these methods do not generally guarantee uniform volume heating.
Induction heating systems based on ferri- and ferromagnetic magnetic materials are, in principle, available but their application to the curing process of composites is limited due to loss mechanisms in magnetic materials and therefore due to the difficulty to control the temperature field.
One attempt to apply indirect induction heating to a part is described in U.S. Pat. No. 6,528,771 B1. U.S. Pat. No. 6,528,771 B1 relates to an induction heating system for fabricating a part by heating and forming the part and a method for controlling an induction heating process. The induction heating system comprises: a susceptor including a susceptor material defining a cavity configured to receive the part, a coil positioned in proximity to the susceptor, and a temperature controller having a power supply and a controlling element. Said susceptor material is configured to respond to electromagnetic flux applied thereto by generating heat so as to increase a temperature of the part in the cavity. The coil is capable of generating the electromagnetic flux when supplied electrical power. Said power supply is operably connected to the coil to supply an amount of the electrical power thereto. Said controlling element is configured to measure trends in output of the power supply and further configured to change the amount of electrical power being supplied so as to control the temperature of the part in the cavity during fabrication based upon the measured trends. U.S. Pat. No. 6,528,771 B1 describes a fixed range of temperature control over a 20° F. window around the Curie temperature.
Accordingly, there is a need for a flexible technique for controlling a process temperature for induction heating of a workpiece.